Four fire engines and around 25 firefighters were called to a fire at a disused sports pavilion in Watling Park on Cressingham Road in Edgware.
Part of the roof was destroyed by fire. There were no reports of any injuries.
The Brigade's 999 Control Officers took 14 calls to the blaze.
The Brigade was called at 1854 and the fire was under control by 1946. Fire crews from Mill Hill, Stanmore and Hendon fire stations were at the scene.
The cause of the fire is under investigation by the Brigade and the Metropolitan Police Service.